1. Field of the Invention
The present invention relates to an image processing apparatus that executes an image process, such as a halftoning process, in an image forming apparatus.
2. Description of the Related Art
Recently, an apparatus, which is typified by a digital multi-function peripheral (MFP), is steadily gaining in popularity. This apparatus totally handles a variety of digitized image data of, e.g. a copier, a printer, a scanner and a FAX, temporarily stores these data in a memory medium such as an HDD, and reuses the data. Very important development items in this type of system are the realization of an architecture that can handle various data easily and seamlessly, and the reduction in cost of the system itself.
On the other hand, within the MFP, practical use has widely been made of an image forming apparatus using an electrophotographic process system such as a laser printer that drives a laser on the basis of image data and executes tone reproduction. In addition, color image forming apparatuses, which repeat the above process for three colors or four colors or reproduce a color image by a tandem process, have become increasing popular. With these developments, the resolution of these output apparatuses has been increasing in recent years. For example, resolutions of, e.g. 600 dpi and 1200 dpi are realized.
In general, when a color image is handled, a monitor or the like, which is a part of a computer, treats the image by an 8-bit RGB system for each color, and a printer or the like treats the image by an 8-bit CMY system or CMYK system for each color. In usual cases, the monitor purely displays 1 pixel with multi-value gray levels by brightness modulation for each of RGB. On the other hand, the output apparatus, which is typified by a printer, employs in a final stage a pseudo-tone (halftoning) process, such as a dither method or a density pattern method, which uses a threshold matrix. Basically, a binary output, which corresponds to ON/OFF of a dot in a pixel unit or a smaller divided pixel unit, is controlled in a predetermined small area. Thereby, the tone is expressed on the basis of the area. In the case of a color image, this process is applied to four colors of C (cyan), M (magenta), Y (yellow) and K (black), or three colors of C (cyan), M (magenta) and Y (yellow). On a final print surface, 3-color or 4-color micro-dots are overlaid, and a fine color tone image is reproduced. Actually, however, K that is produced by overlaying color materials of CMY has no ideal color characteristics of K. Thus, most of conventional color image output apparatuses are based on CMYK.
A document, graphic, photo, or the like, which is created and edited by a host computer or a personal computer, is converted to a PDL (e.g. Postscript or PCL) by a printer driver. The converted data is sent to an image output apparatus via relay means such as a LAN or a USB. On the image output apparatus side, a controller section in the image processing apparatus interprets the language of the data and develops it into raster data (RIP processing).
In general, a halftoning process is executed in the controller section. Image data that is output from the controller has a tone reproduction capability corresponding to the printing performance of the output apparatus, for example, 1 bit to 8 bits per pixel for each color. Basically, the number of bits of the output data is designed in consideration of the performance of the engine. Alternatively, the halftoning process may not be executed in the controller, and tone reproduction may be executed by applying an engine-side laser modulation technology (e.g. pulse width modulation technique or power modulation technique). In principle, from the standpoint of image quality, the possibility that a high-quality image is reproduced becomes higher as the number of bits in the RIP increases.
In the output apparatus such as an electrophotographic apparatus, output print characteristics (e.g. dot gain) have a large distortion relative to ideal linear characteristics due to the complexity of the process. In this respect, the halftoning process is very important, and the image quality varies to a great degree depending on design specifications including an algorithm and parameters. The halftoning process has a great influence on the image quality, and various algorithms have conventionally been proposed.
For simple description, algorithms for the halftoning process are classified into AM modulation (cyclic modulation) and FM modulation (frequency modulation). In the AM modulation, the size of a basic halftone dot is modulated at each gray level on the basis of a given cycle, angle and shape, and thus the tone is reproduced. In the FM modulation, a mean distance between fixed-sized halftone dots is varied at each gray level, and thus the tone is reproduced. In general, in some cases, an error diffusion process may be regarded as a kind of the FM modulation.
When these methods are applied to the output apparatus such as the electrophotographic apparatus, the following problems will arise. In the electrophotographic apparatus, it is difficult to form an independent 1 pixel (e.g. 1 pixel of 600 dpi) in a stable state with the same resolution as the printer (i.e. 1 dot of 600 dpi is reproduced by the printer of 2400 dpi). In general, if an FM modulation type dither matrix is used, a high-quality image cannot be obtained.
Thus, by using an AM modulation type dither matrix, the tone is reproduced on the basis of the area in units of a plurality of pixels combined. Thereby, a stable image is obtained. AM modulation type dither methods include various shape-type methods, such as a half-tone dot type, a line type and a chain type. These methods, however, are essentially the same in that a plurality of dots are gathered in a given direction, thereby reproducing tone.
In this case, in order to increase the number of pseudo-gray levels to such a level as to reproduce a visually satisfactory image, it should suffice if the basic size of each halftone-dot of a threshold matrix is increased. However, the resolution decreases as the basic size of each halftone-dot becomes greater. In the halftoning process, the resolution characteristics and the tone characteristics are contradictory. In the case where this halftoning process is executed, even if the tone is satisfactory, the image quality of an edge part of a gray-level character or line in the image, which represents resolution information, considerably deteriorates.
The electrophotographic apparatus does not have a resolution of several-thousand dpi, which is a resolution level of the printer. A currently dominant type of electrophotographic apparatuses have resolutions of about 600 dpi to 1200 dpi at most. There are too many geometrical restrictions in order to obtain satisfactory tone reproduction by creating halftone-dots with such a resolution and a given angle and line number (about 100 to 200 lines). It is not impossible to forcedly form a screen, ignoring a geometrical position error in digital arithmetic operations. However, if the halftoning process is executed using a threshold matrix that is formed in this way, an image that is formed on a final print surface includes, as a matter of course, geometrical errors of many halftone centers at locations on the two-dimensional plane. Consequently, textures, which are unsightly, occur at given gray levels, and graininess increases.
In order to solve these problems, there has been developed an image forming apparatus such as an electrophotographic apparatus, wherein a technique of PWM (Pulse Width Modulation), for instance, is used, and multi-value image data is generated by subjecting input image data to a multi-value dither process, etc. Using the multi-value image data, the output area within 1 pixel is modulated, and the inside of the 1 pixel is expressed with several gray levels.
In the image forming apparatus capable of printing multi-value image data, various image processes, such as a color conversion process, an UCR (under-color removal) process and a gamma correction process, are executed. Thereafter, in order to reproduce the prescribed number of gray levels, which is inherent to the printer engine, a multi-value pseudo-tone process is executed for each color. Thereby, multi-value image data of 1 pixel bit is obtained. The image reproducibility is enhanced by concentrating a more information amount in 1 pixel. However, the fact that the number of bits that are handled at 1 pixel means that the data amount to be handled increases accordingly. Even if the technology is constantly developing, the amount of multi-value data, which is treated with the resolution of the output apparatus such as the electrophotographic apparatus, is enormous, and a serious problem arises with the cost of, e.g. a memory load.
Under the circumstances, in modern output apparatuses, multi-value image data is not handled as such. In some cases, data is compressed by some compression process at a given point along the image path, and thus the performance of the entire system is improved.
It is necessary to treat various images, such as a text, graphic and photo, as print-out images. Each object has different important characteristics. For example, a text needs to have good resolution characteristics such as edge reproducibility, a photo needs to have good tone characteristics, and a photo needs to have both good characteristics. As mentioned above, a single halftoning process cannot meet the requirements for both characteristics. Thus, an optimal screen is individually used for each of objects, and the image quality is enhanced.
On the other hand, the functions of the MFP include, aside from the printer function, a PC-FAX function, a BOX function, etc. It is preferable to seamlessly use data between the functions. For example, an image to be handled in the FAX function is of 1 bit/pixel, according to the standard of the FAX function. In the PC-FAX function, too, it is necessary to treat a PIP-processed image with 1 bit/pixel. Images to be handled in the BOX function have different bit numbers, depending on uses, and there is a case in which a data format that is compressed by a general-purpose scheme, such as JPEG, is preferable.
For example, when the printer is used, the image quality can be greatly improved. For this purpose, it is preferable to treat an image with multi-bit/pixel. Depending on the functions, the proper bit number for 1 pixel differs. Some data conversion is needed in a case of reusing an image, which is stored in a hard disk drive or the like, for another function. There is a problem of complexity in data handling.
There are known U.S. Pat. No. 5,949,964 and Jpn. Pat. Appln. KOKAI Publication No. 2003-234900, which disclose techniques for solving the above problems.
In U.S. Pat. No. 5,949,964, rasterized image data is encoded along with a discrimination signal for discriminating an object, and is temporarily stored in a memory device. The encoded image data and discrimination signal, which are retrieved from the memory device, are decoded, and the decoded image data is subjected to a halftoning process on the basis of the similarly decoded discriminating signal. Thus, an optimal halftone output for each object is obtained. In addition, since a data compression process is executed at a point along the image path, the capacity of the memory device or the like can be reduced.
In general, reversible compression is desirable as compression of image data that is exactly rasterized by a controller. The controller has to handle complex and various objects, and images that are formed by rasterizing these objects must essentially be reproduced with precision. Basically, except for some kinds of photo images, it is necessary to avoid occurrence of unexpectable noise due to compression of a given object, which may lead to serious degradation in image quality.
In a case where 8-bit multi-value image data is handled, for example, if the image data is compressed by irreversible compression such as JPEG, the reproduction performance for high-frequency image data is very low. Thus, such irreversible compression is not suited to objects such as texts and fine graphics. On the other hand, when reversible compression is executed, the code amount of a natural picture such as a photo cannot sufficiently be reduced, contrary to expectation, and a very large memory is needed. Thus, there is the problem that the advantageous effect of the system configuration is small, compared to its complexity.
It is possible to think of a method in which an optimal compression method is switched in accordance with the property of each object. This method, however, requires very complex processing, and the cost effectiveness of the system configuration is low.
Furthermore, the image data and the object discrimination signal have to be compressed in the path of the compression process. Consequently, there arises such a problem that the process tends to become complex and the data reduction effect by the compression is low.